FIGS. 3(a) to 3(d) illustrate the construction and operation of a prior art semiconductor laser device. In FIG. 3, reference numeral 1 designates a semiconductor light waveguide path which generally corresponds to an active region. Reference numberals 2 and 3 designate coating layers at the cavity facets, and dielectric thin films are generally used therefor. Light is amplified while traveling through the light waveguide path 1. When the light reaches the facet 2, a portion thereof is emitted to the outside and the remaining portion thereof is reflected and again amplified while traveling through the light waveguide path 1. When the light reaches the facet 3, a portion thereof is emitted to the outside and the remaining portion thereof is again reflected and travels through the light waveguide path 1.
When the width d of the light waveguide path 1 shown in FIG. 3 is wide and a primary mode or a secondary mode of oscillation occur instead of the fundamental mode (0 order mode), the oscillation mode becomes unstable. The stability is attributed to a lack of uniformity in the; thickness and composition of the light waveguide path, the injected current density, and the temperature rise. Thus, in some cases a fundamental mode oscillation is produced but in other cases a primary order mode or a secondary order mode are obtained.
FIG. 3(c) shows an electric field distribution (transverse mode) of the zero order, the primary order, and the secondary order oscillation mode in the light waveguide path.
When the light waveguide path is widened to about 100 microns in order to obtain a high power output, a variety of higher order modes oscillate at the same time, and then the far-field radiation pattern is complicated and may have a plurality of peaks as shown in FIG. 3(d).
When the width of the light waveguide path is widened so that the higher order transverse oscillation modes are produced to obtain a high output, the laser beam cannot be focused to a point. Furthermore, the ratio of the light which is incident on an optical system to the total light output is low. Even in the same laser device the transverse oscillation modes vary and the laser behavior varies to a great extent because variations in the injection current and the temperature, whereby no linearity and no reproducibility of the characteristics are obtained.